This invention relates to medical diagnostic ultrasound imaging, and more particularly to a method and apparatus for acquiring ultrasound fields of view to generate three-dimensional ultrasound volumes.
Medical diagnostic ultrasound systems are commonly used to generate two-dimensional diagnostic images of internal features within a patient's body. To do so, a sonographer positions an ultrasound transducer probe adjacent to a patient's target area. The probe is a non-intrusive device including an array of acoustic transducer elements. The transducer elements emit ultrasonic energy at a frequency on the order of 2.0 MHz to 10 MHz. The transmitted ultrasound energy propagates into the patient where it is in part absorbed, dispersed, refracted, and reflected by internal structures. Reflected ultrasound energy is received back at the transducer probe where it is converted back into electronic signals. Body tissues, for example, appear as discontinuities or impedance changes, and are indicated by the converted electronic signals.
Converted electronic signal samples undergo beamforming to correlate the samples in time and space to a patient's target area. Exemplary beamforming parameters for controlling the imaging process include focus, steering, apodization and aperture. Focus is a time delay profile of active transducer elements. Steering is the control of focus depth points along azimuth and elevation axes of a transducer probe scan. Apodization is a voltage weighting profile of active transducer elements. Aperture is the control of the number of transducer elements which are active along azimuth or elevation axes of the transducer probe for a given scan. The beamformed data are processed to analyze echo, Doppler, and flow information and obtain a cross-sectional image of the patient's targeted anatomy (e.g., tissue, flow, Doppler).
A conventional image is a brightness image (i.e., referred to as a `B-mode image`) in which component pixels are brightened in proportion to a corresponding echo sample strength. The B-mode image represents a cross section of the patient target area through a transducer's scanning plane. Typically the B-mode image is a gray scale image in which the range of lighter to darker gray-scale shades correspond to increasing brightness or echo strength. The typical ultrasound B-mode image is formed by a linear scan or sector scan of the patient's target area by the transducer probe. The individual images produced by ultrasound imaging systems include discrete frames. A frame is a tomographic slice which represents a single sample in elevation. For a given frame in which first active transducer elements transmit an ultrasound signal and second active transducer elements receive an ultrasound echo, the transducer probe defines a given field of view. Such field of view depends on the number of active transducer elements, the relative spacing of the elements and the steering and focus of each element. Each frame has a limited field of view due to a relatively narrow region traversed by the transmitted ultrasound energy. The frame is a single tomographic slice having a single elevation sample. As the transducer probe is manipulated along the patient's body surface, each previous image slice is replaced on the viewing display by a new image slice defined by the current position, and thus field of view, of the transducer probe.
Given the generally narrow field of view of conventional ultrasound systems, it is desirable to extend the field of view to acquire images over large portions of the patient anatomy. Increasing the size of transducer or the number of transducer elements are alternative approaches. However, such approaches adds significant hardware expense. Also, increasing the number of transducers increases processing overhead cost. Another approach is to compound images from the scanning process into a larger image. Previously, it has been demonstrated that a real-time compounded ultrasound two-dimensional image could be generated by using so-called compound B-scanners. These B-scanners use a transducer mounted on an arm assembly that constrains the transducer to move along a single plane or axis. Either the arm assembly or the transducer element itself is provided with sensing devices for tracking the precise position of the transducer. This positional information then is used to register each one of discrete image frames into a composite image. An example of an arm assembly is disclosed in U.S. Pat. No. 4,431,007 to Amazeen et al. for Referenced Real-Time Ultrasound Image Display.